Backup power for emergency lighting fixtures such as emergency lights and exit signs has conventionally been provided by rechargeable batteries that are “trickle” charged when an alternating current (AC) line voltage is available. Generally, when the AC line voltage is lost during a power outage, for example, the batteries are used to supply power to light sources of emergency lighting fixtures. Typically, batteries are charged slowly to increase their life expectancy and avoid damage. Further, because of material limitations of general purpose rechargeable batteries, the batteries cannot be charged quickly even if desired. Thus, complete charge times for batteries in emergency lights and exit signs may be from about one to seven days. Even when several battery powered emergency lights are installed in a building, it is possible to charge the batteries in each of the emergency lights simultaneously without overloading the electrical distribution system of the building, because each of the emergency lights draws a relatively low amount of power for “trickle” charging.
Based on advances in materials science, supercapacitors stand to replace batteries as a power storage means in emergency lighting fixtures. Electric double-layer capacitors (EDLC) or supercapacitors (hereinafter referred to generally as supercapacitors) offer advantages and drawbacks over general purpose rechargeable batteries. For example, supercapacitors can be charged quickly without decreased lifetime expectancy or causing damage. Projected charge times for supercapacitors range from seconds to minutes for a full charge, as compared to hours or days for rechargeable batteries. For a comparable amount of power storage, the relatively quick charge time of a supercapacitor is attributed to a relatively high current draw, as compared to the “trickle” charge current draw for a rechargeable battery.
The relatively high current draw of supercapacitors presents a problem in many buildings such as office buildings, stores, shopping malls, theaters, and hospitals, for example, which generally include several backup powered emergency lighting fixtures. That is, if starting from a fully discharged state, the current draw required to charge several supercapacitor-powered emergency lighting fixtures may overload a building's power distribution system. Thus, to accommodate supercapacitor-powered emergency lighting fixtures, branch, feeder, and possibly even service circuits may need to be redesigned, retrofitted, or replaced to accommodate the large increase in peak current to charge the fixtures.
After several supercapacitor-powered emergency lighting fixtures are fully charged, their current demand returns to a normal level. However, a building's power distribution system would need sufficient capacity to handle the peak current demand required to simultaneously charge several discharged supercapacitors, even if the peak demand were expected to last only a few minutes. Thus, more robust and expensive wiring and distribution panel boards would be required to handle the supercapacitor charging energy spike (KVA peak demand), but would be unnecessary otherwise.